The instant invention relates to an electron source for an electron gun.
In the prior art, two main types of electron guns are used, sorted according to the type of the electron source, that is, those provided with a thermoelectronic source and those provided with a plasma source.
In the electron guns with a thermoelectronic source, electrons are created by a solid heated at a high temperature. The main drawback of this type of device is the relatively low lifetime of the filament. In order to increase the lifetime, it is necessary, on the one hand, to provide for very sophisticated materials used for example in the cathodes of the electronic tubes and, on the other hand, to reduce as much as possible the pressure in the electron generation area. One thus tries to obtain pressures lower than 10.sup.-5 Torr (about 10.sup.-3 Pa), which requires very sophisticated vacuum equipment.
FIG. 1 schematically shows an electron gun with a plasma source. This electron gun comprises a cathode plate K positioned in front of an anode plate A. The electrical field and gas pressure conditions between the cathode and the anode are chosen for obtaining a glow discharge in the area included between those plates. In practice, this means that there must be a minimal field in the range of a few hundreds V/cm and a pressure in the range of 1 to 10 Pa. Thus, a plasma P is generated between the cathode and the anode and the electrons hit the anode plate. This anode plate A is provided with a central aperture through which the electrons can escape; they are then accelerated by various means towards a collecting plate C. For obtaining an accelerating area with a sufficient length, the pressure in the area included between the anode plate A and the collecting plate C must be sufficiently low. For example, for obtaining a free average path of the electrons of 20 cm, the pressure must be lower than 0.1 Pa.
The structure of FIG. 1 is schematically drawn. In practical devices, numerous improvement are provided, for example intermediary electrodes between the cathode and the anode. Those electrodes can be of the grid type. Similarly, for maintaining a good focussing of the electron beam in the accelerating area, magnetic fields parallel to the electron propagation direction are sometimes provided.
Those plasma electron sources present, with respect to the thermoelectronic sources, the advantage of providing a larger lifetime and an operating ability at a higher pressure (1 to 10 Pa instead of 10.sup.-4 to 10.sup.-3 Pa).
However those plasma sources still present several drawbacks.
A first drawback is due to the fact that, a plasma being generated in the whole area included between the cathode and the anode, the device yield is necessarily lower than the unit since a portion of the electrons will hit the anode plate A. This loss can be lowered by using, instead of a plain aperture as schematically shown in FIG. 1, a transparent anode system provided with a magnetic focussing. However, yields lower than the unit are still obtained, for example in the range of 70%.
Another drawback is due to the fact that the plasma generation area between the cathode and the anode and the accelerating area between the anode A and the accelerating plate C must necessarily have different pressures: the second area must have a lower pressure than the first one. Therefore, sophisticated differential pumping systems are to be used for optimizing the pressures in each area. This is usually carried out by injecting a gas into the cathode-anode area and by pumping in the accelerating plate-anode area.
Thus, an object of the instant invention is to provide for a new type of plasma electron source palliating the hereinabove mentioned drawbacks of the conventional plasma electron sources.